Article: Cleaning glass: A many-faceted issue Author(s): Stephen P. Koob Source: Objects Specialty Group Postprints, Volume Eleven, 2004 Pages: 60-70 Compilers: Virginia Greene and Patricia Griffin th © 2007 by The American Institute for Conservation of Historic & Artistic Works, 1156 15 Street NW, Suite 320, Washington, DC 20005. (202) 452-9545 www.conservation-us.org Under a licensing agreement, individual authors retain copyright to their work and extend publications rights to the American Institute for Conservation. Objects Specialty Group Postprints is published annually by the Objects Specialty Group (OSG) of the American Institute for Conservation of Historic & Artistic Works (AIC). A membership benefit of the Objects Specialty Group, Objects Specialty Group Postprints is mainly comprised of papers presented at OSG sessions at AIC Annual Meetings and is intended to inform and educate conservation-related disciplines. Papers presented in Objects Specialty Group Postprints, Volume Eleven, 2004 have been edited for clarity and content but have not undergone a formal process of peer review. This publication is primarily intended for the members of the Objects Specialty Group of the American Institute for Conservation of Historic & Artistic Works. Responsibility for the methods and materials described herein rests solely with the authors, whose articles should not be considered official statements of the OSG or the AIC. The OSG is an approved division of the AIC but does not necessarily represent the AIC policy or opinions. AIC Objects Specialty Group Postprints, Volume 11, 2004 CLEANING GLASS: A MANY-FACETED ISSUE Stephen P. Koob Abstract The removal of decades of cigarette smoke, grease and grime from handling, and more recently, pollution and off-gassing from improper storage cabinets, will contribute significantly to the prolonged stability of all glasses. In addition, glasses that are subjected to prolonged storage in high humidity (over 55% rH) will begin to hydrate and the alkali in the glass is brought to the surface. Unless this is removed, the alkali will eventually start to dissolve the silica in the glass. There are numerous types of glasses, but the “silica-soda-lime” glasses make up approximately 90% of all types from antiquity to the present, from Roman vessels to modern window panes. These glasses are generally thought to be very stable, but they will slowly deteriorate (through weeping or crizzling) over decades and centuries of prolonged exposure to high humidity, grime and pollution. Most glasses can be washed using a detergent and water, as long as the glass is sound and in good condition. Warm tap water can be used for the initial washing, followed by rinsing with deionized or distilled water. A dilute conservation-grade detergent is recommended, such as Synperonic A-7 or Triton XL-80N. Cleaners or detergents that contain perfumes or ammonia should be avoided. A simple washing, even once in the lifetime of a glass, will protect the glass for decades, if not centuries. Soft brushes and soft cotton toweling are recommended. Glass objects should never cleaned in a dishwasher, or with abrasive sponges or cleaners. Introduction There are many, many reasons to wash glass, the most basic of which is that the glass looks much better when clean (Figs. 1, 2). There are also many instances and situations when one should not clean glass, as in the case of severely weathered archaeological glass, where cleaning will remove the iridescent or opalescent weathering layers (Fig. 3). The primary reason not to remove the weathering on archaeological glass is the fact that glass deteriorates, or corrodes, or “weathers” from the outside in. If the weathering is removed, so is the original surface, including the details, decoration and information preserved in that surface layer (Smith 1999). The end appearance may be a glass with its original color, but since glass does not always corrode evenly, the result will be a glass with a severely pitted and etched surface, whose integrity has been compromised. Unfortunately, every museum and every time period have their own tastes, and until recently it was not unusual for archaeologists and curators to remove the weathering layers from glazed ceramics and glasses. However, this is not an acceptable practice today. 60 Koob AIC Objects Specialty Group Postprints, Volume 11, 2004 Figure 1. Dirty glass plate with paper label Figure 2. Plate in Fig. 1, after washing. and masking tape (CMOG 59.4.279A). Figure 3. Roman glass unguentarium with flaking Figure 4. Well-preserved glass weathering. Arthur M. Sackler Gallery, Smithsonian bracelets from a 7th century BC Institution (LTS1985.1.174.24). tomb at Gordion, Turkey (Gordion Inventory G 162). Some archaeological glasses do not even need to be washed, or can be safely washed, as they have no weathering, and come out of the ground looking like they were made yesterday (Fig. 4). Some modern studio glasses should never be washed as they have cold painted decoration (applied after the glass was made and cooled) or organic attachments (textiles, fur, wood, etc.). These pieces should be treated as other sensitive composite materials, e.g., ethnographic objects. 61 Koob AIC Objects Specialty Group Postprints, Volume 11, 2004 Cleaning The first and most important point in cleaning glass is to know something about the glass. If the glass is sound and in good condition, it generally can be cleaned, or if the glass has been previously cleaned, it can be cleaned again. If there is some doubt, one should learn more about the glass. Most glasses are of a composition that are called silica-soda-lime glasses, with a very stable composition of approximately 70% silica, 20% soda (sodium carbonate, one of the most common alkalis used in the production of glass), and 10% lime (or CaO), which serves as a stabilizer. This composition makes up about 90% of all glasses, and has changed very little from ancient times to modern. Despite the many new types of glass being developed every day, most glasses from Roman times to modern window glasses have virtually the same composition. The glasses are strong, durable and can safely be cleaned. Additional reasons for cleaning glass include the removal of accumulated dirt and grime, tapes, adhesive labels (see Fig. 1), old or yellowed adhesives or fills (Fig. 5), or to repair the results of a natural disaster (Fig. 6). In addition, almost any glass made before 1980 probably has some traces of cigarette smoke or nicotine on it. Smoking was so commonplace 25 years ago that it was even possible to smoke in most museums. Figure 5. Yellowed adhesive and fill on a Figure 6. Chandelier, damaged and mud- glass sculpture (CMOG 59.4.426). covered after the Corning flood of 1972 (CMOG 60.2.39). Materials and Techniques for Cleaning The removal of adhesives, tapes, and labels involve well-published conservation treatments (Navarro 1997), but basic cleaning is a different problem. Glasses that are in good condition and are not sensitive to water (see above), can easily and efficiently washed with soap and tap water. Ideally, washing should be done in a plastic sink (Fig. 7). Polypropylene sinks are available, and a standard size is 30” long x 24” wide x 8” deep, not counting the 24 x 30 inch drainboard (available from McMaster-Carr, see Suppliers). The sink should also have a long gooseneck faucet with wide handles, and even a separately plumbed sprayer. A supply of pure water is 62 Koob AIC Objects Specialty Group Postprints, Volume 11, 2004 required, and a simple solution to this is to place close to the sink a large polypropylene tank attached to a deionizing column (see Fig. 7). Figure 7. Polypropylene sink, with sprayer on the left, deionized water tank on the right. The large sink provides ample working space, so that even large glasses can be completely washed and rinsed. A plastic sink will not prevent glass from breaking if it is dropped heavily, but it will minimize the risk. Polypropylene is certainly better than stainless steel, where one tap against the side, even a gentle one, will break most glasses. If it is not possible to purchase a plastic sink, one should at the very least use a large shallow plastic bucket or tub inside a metal sink. Gloves should not be worn when washing glass (or even handling most dry glass, with the two exceptions of acid-etched and most pâte-de-verre glasses, which are extremely sensitive to staining from oils). Glass is slippery even when dry; once covered with soap it is even more slippery. A conservation grade detergent should be used, such as Triton XL-80 (see Suppliers). The detergent should be diluted with water to approximately 15:1, but the dilution ratio is not critical. Other clear liquid detergents may be used, but one should avoid powdered detergents, and anything that has color, perfumes and/or ammonia. Cleaning can be done, by rinsing, dipping, brushing, or swabbing. This depends on the shape of the glass, and how strong/stable it is. A Venetian goblet is probably best cleaned with soft toweling, either a cut-up 100% cotton diaper or soft paper toweling, both of which are even softer when wet. Brushes or small bits of cut-up sponges are also useful, as is a large curved bottle brush with very soft, natural soft bristles and a plastic handle (available from Fisher Scientific, see Suppliers). Alternatively, a soft paintbrush may be used, but one should be sure to tape over the metal ferrule. Straight brushes are more difficult to use, as one needs to keep the bristles aimed at what is being cleaned. Cleaning should begin with warm tap water, brushing or swabbing with soap, followed by rinsing in tap water.